Why Do B12 Levels Go Down? Unpacking the Root Causes

November 18, 2025 •

3 min read

According to the NIH, up to 6% of the U.S. population is deficient in vitamin B12, with a much higher prevalence in older adults and specific groups. A shortage of this vital nutrient, also known as cobalamin, can cause a range of serious health issues, which is why understanding why do B12 levels go down is crucial.

Quick Summary

Low B12 levels stem from insufficient dietary intake, impaired absorption due to medical conditions, or drug interactions. This deficiency can lead to anemia and neurological problems if left unaddressed. It is critical to identify the root cause for proper treatment.

Key Points

Inadequate Absorption: The most common reason for low B12 is the body's inability to properly absorb the vitamin, often due to conditions affecting the stomach or intestines.
Dietary Factors: People on strict vegan or vegetarian diets are at high risk unless they consume fortified foods or supplements, as B12 is mainly found in animal products.
Pernicious Anemia: An autoimmune condition called pernicious anemia prevents the production of intrinsic factor, a protein necessary for B12 absorption.
Medication Side Effects: Certain medications, such as metformin and acid-reducing drugs (PPIs), can interfere with B12 absorption with long-term use.
Age-Related Decline: Older adults are more susceptible to low B12 due to decreased stomach acid and an increased risk of malabsorption issues.
Underlying Health Conditions: Digestive disorders like Crohn's disease, celiac disease, and gastric surgeries can all lead to poor B12 absorption.

Understanding the Complex Path to B12 Deficiency

Vitamin B12 is essential for nerve function, DNA synthesis, and red blood cell formation. Unlike many vitamins, B12 is not produced by the human body and must be obtained from external sources. The process is complex and relies on several factors, including adequate stomach acid and a protein called intrinsic factor for absorption. A deficiency can occur when any part of this process is disrupted, either by diet, medical issues, or other factors.

Inadequate Dietary Intake

Low B12 often results from a diet lacking sufficient animal-based products, as B12 is found primarily in these foods. Individuals following strict vegan or vegetarian diets face a higher risk if they don't use fortified foods or supplements. B12 can be stored, so symptoms might not appear immediately. A generally unbalanced diet can also contribute to insufficient B12 intake.

Malabsorption Issues in the Digestive System

Even with adequate dietary intake, the body might not absorb B12 properly due to digestive problems, a common cause of deficiency. Conditions like pernicious anemia, atrophic gastritis, gastrointestinal surgery, inflammatory bowel disease (IBD) such as Crohn's, and small intestinal bacterial overgrowth (SIBO) can all impair the process by affecting intrinsic factor, stomach acid production, or the intestinal lining.

Medications That Affect B12 Levels

Certain medications can hinder B12 absorption or metabolism, particularly with long-term use in older adults. This includes Metformin and acid-reducing drugs like Proton Pump Inhibitors (PPIs) and H2 Blockers. Chronic and excessive alcohol intake also damages the digestive system and interferes with B12 absorption.

Comparison of B12 Deficiency Causes


Cause	Mechanism	At-Risk Populations	Treatment Consideration
Dietary Insufficiency	Low intake of animal products or fortified foods.	Vegans, vegetarians, those with poor diet.	Supplementation or dietary changes.
Malabsorption	Failure to absorb B12 in the digestive tract.	Elderly, those with autoimmune diseases (e.g., pernicious anemia), IBD, or post-surgery.	Injections, high-dose oral supplements.
Medication Use	Drugs that interfere with absorption or metabolism.	Diabetics on Metformin, individuals on long-term acid-reducing medications.	Supplementation, monitoring B12 levels.
Genetic Disorders	Rare inherited conditions affecting B12 processing.	Individuals with specific genetic mutations.	Specialized, often higher-dose, supplementation.

Other Notable Causes

Less common factors can also contribute to low B12:

Chronic Alcoholism: Heavy alcohol use impairs nutrient absorption, including B12.
Nitrous Oxide Abuse: Repeated exposure can inactivate B12.
Infections: Certain parasites can compete for B12.
Genetic Factors: Rare inherited disorders can prevent proper B12 processing.

Conclusion

Understanding why do B12 levels go down is crucial for managing and preventing this deficiency. Causes range from diet to complex medical conditions. Malabsorption is a significant factor, especially in older individuals. Treatment depends on the cause; injections may be necessary for those with absorption issues. Consulting a healthcare provider for diagnosis is vital, as symptoms can be subtle. Identifying the root cause ensures proper treatment and helps prevent serious neurological issues. More information is available from resources like the {Link: NIH Office of Dietary Supplements https://ods.od.nih.gov/factsheets/VitaminB12-Consumer/} and {Link: MSD Manuals https://www.msdmanuals.com/home/disorders-of-nutrition/vitamins/vitamin-b12-deficiency}.

Frequently Asked Questions

The most common reason for B12 deficiency, especially in older adults, is malabsorption, where the body cannot properly absorb the vitamin from food. This can be caused by conditions like pernicious anemia or low stomach acid.

Yes, a vegan diet can lead to low B12 levels because the vitamin is found naturally almost exclusively in animal products. Vegans must consume fortified foods or supplements to ensure adequate intake.

Stomach problems like atrophic gastritis and pernicious anemia can reduce the production of stomach acid and intrinsic factor, both of which are critical for the absorption of B12 from food.

Several medications can lower B12 levels, including metformin for diabetes and long-term use of acid-reducing drugs like proton pump inhibitors and H2 blockers.

Older adults are at higher risk for low B12 because they are more prone to developing conditions like atrophic gastritis, which reduces stomach acid needed for B12 absorption.

Early signs of a B12 deficiency can be subtle but often include fatigue, weakness, a sore tongue, and neurological symptoms like tingling or numbness in the hands and feet.

Yes, chronic and excessive alcohol consumption can damage the digestive system, impairing the absorption of nutrients like B12 and leading to a deficiency.